1. Field of the Invention
The present invention relates to a method of detecting an abnormality in polishing of a substrate, such as a wafer, and a polishing apparatus. More particularly, the present invention relates to a method of detecting an abnormality in a process of polishing a peripheral portion of a substrate with a polishing tool, such as a polishing tape or a fixed abrasive.
2. Description of the Related Art
A polishing apparatus, which has a polishing tool (e.g., a polishing tape or a fixed abrasive), is used for polishing a peripheral portion of a wafer. This type of polishing apparatus is configured to bring the polishing tool into contact with the peripheral portion of the wafer, while rotating the wafer, to thereby polish the peripheral portion. In this specification, the peripheral portion of the wafer is defined as a region including a bevel portion which is the outermost portion of the wafer and a top edge portion and a bottom edge portion located radially inwardly of the bevel portion.
FIG. 31A and FIG. 31B are enlarged cross-sectional views each showing the peripheral portion of the wafer. More specifically, FIG. 31A shows a cross-sectional view of a so-called straight-type wafer, and FIG. 31B shows a cross-sectional view of a so-called round-type wafer. In the wafer W shown in FIG. 31A, the bevel portion is an outermost circumferential surface of the wafer W (indicated by a symbol B) that is constituted by an upper slope (an upper bevel portion) P, a lower slope (a lower bevel portion) Q, and a side portion (an apex) R. In the wafer W shown in FIG. 31B, the bevel portion is a portion B having a curved cross section and forming an outermost circumferential surface of the wafer W. The top edge portion is a flat portion E1 located radially inwardly of the bevel portion B. The bottom edge portion is a flat portion E2 located opposite the top edge portion and located radially inwardly of the bevel portion B. The top edge portion E1 and the bottom edge portion E2 may be collectively referred to as edge portion. The edge portion may include a region where devices are formed.
In a fabrication process of SOI (Silicon on Insulator) substrate, there is a need to form a vertical surface and a horizontal surface on the edge portion of the wafer W, as shown in FIG. 32. A cross section of such edge portion is achieved by a polishing process as illustrated in FIG. 33. Specifically, while the wafer W is rotated, a right-angle edge of a polishing tool 400 (e.g., a polishing tape or a fixed abrasive) is pressed against the edge portion of the wafer W to thereby polish it. The polishing tool 400 has its lower surface serving as a polishing surface that holds abrasive grains thereon. This polishing surface is disposed in parallel with the wafer W. In this state, the polishing surface of the polishing tool 400 is pressed against the edge portion of the wafer W to thereby form a right-angle cross section, i.e., the vertical surface and the horizontal surface, on the edge portion of the wafer W.
However, the edge portion of the wafer may not be polished properly due to some causes, such as aged deterioration of the polishing tool 400 or a malfunction of the polishing apparatus. Such an abnormality in polishing of the wafer would result in a lowered yield. Therefore, it is necessary to detect the abnormality in polishing of the wafer promptly. However, whether or not the edge portion of the wafer is polished properly cannot be judged until the cross section of the edge portion of the wafer is examined under a microscope.